This invention relates to power transmission changeover devices for four-wheel drive vehicles, and particularly a power transmission changeover device having a sub-transmission.
Four-wheel drive vehicles are classified into part-time 4WD's, in which four-wheel (4WD) and two-wheel (2WD) drive modes are changed over by manual operation of a lever or switch, and full-time 4WD's, in which driving torque distribution to the front and rear wheels is automatically adjusted according to the difference in rotation speed between the front and rear wheels.
Many part-time 4WD's have, besides the main transmission, a sub-transmission adapted to change over between a high position for a normal 4-WD travel and a low position for traveling over rough or sandy roads. The sub-transmission includes a gear train for changing over the driving force transmission path, and constitutes a part of the power transmission changeover device for changeover between 4WD and 2WD.
The sub-transmission is changed over by the driver by operating a lever or switch with the main transmission in the neutral position to shut off the torque from the engine.
To change over the position of the sub-transmission, it is moved from the high-gear or low-gear position to the neutral position, and then to the low-gear or high-gear position. In the case of an automatic transmission (hereinafter abbreviated to "AT") vehicle, gears may rattle during such gear position changeover.
In AT cars, slight torque tends to be transmitted to the output shaft of the transmission even when it is in the neutral position. Thus, if no brakes are applied, the vehicle will begin to move slowly (creep). Such creeping is especially remarkable when it is cold because oil in the transmission is high in viscosity in a cold environment.
As described above, the sub-transmission is changed over with the main transmission in the neutral position. In the case of an AT car, during a very short period while the gear position of the sub-transmission passes through the neutral position, gears of the sub-transmission provided at the output side of the main transmission rotate due to creeping of the vehicle. This makes smooth meshing of new gears difficult. Gears thus tend to rattle.
An object of this invention is to provide a power transmission changeover device for an automatic-transmission, four-wheel drive vehicle which produces no rattling of gears while the gear position of the sub-transmission is being changed over.
In this type of actuator of a power transmission changeover device having a diaphragm, it is necessary to keep sucking air to maintain the new position even after the drive mode has been changed over between 4WD and 2WD or between high and low.
Thus, while the drive mode is held in any of these positions, a load keeps acting on the thin diaphragm, thus lowering its durability. Also, while air is being sucked, the negative pressure from the engine manifold tends to fluctuate, thus decreasing the reliability of the actuator.
Thus another object of this invention is to provide a power transmission changeover device for a four-wheel drive vehicle which can improve durability and reliability of its actuator.